Hydrogen burns cleanly in air, producing water as a "waste" product. Powering vehicles, machinery or appliances with hydrogen powered engines or fuel cells eliminates the air pollution associated with fossil fuel powered engines.
But the heat value per volume of hydrogen is very low compared to fossil fuels like gasoline. The heat value per volume can be increased by placing the hydrogen under thousands of pounds per square inch of pressure, cooling it to a liquid, or absorbing it into a solid such as a metal hydride. Pressurizing or liquefying hydrogen requires bulky, expensive processing and storage equipment. It can also be dangerous. For instance, if liquid hydrogen is heated, it converts to a gas. This may significantly raise the pressure within its storage device, with possible drastic consequences.
Placing hydrogen in a solid form avoids these problems. Storing hydrogen as a solid has many advantages. For instance, volumetric hydrogen density in a solid such as a metal hydride is fairly high, about the same as liquid hydrogen, making metal hydride a compact storage medium. And binding the hydrogen as a solid means it will not desorb unless heat is applied, thereby improving safety.
Metal hydrides are heavy, however. The gravitational density of hydrogen is very low. This means the amount of energy per weight of metal hydride may be low compared to fossil fuels.
Metal hydride also creates several engineering problems. For instance, metal hydride tends to break down into fine particles that can plug a gas filter. The hydride particles expand and contract upon hydrogen absorption and desorption, respectively. This may cause densely packed metal hydride beds to form that when they expand may damage the container holding the metal hydride. Adding or removing heat is necessary to the hydrogen absorption and desorption processes; hydride powders transfer heat poorly, however.
Attempts to overcome these problems have been made. Some metal powders, like copper or aluminum powders, have been used as a binder to help hold pressed metal hydride powder in compacts. But the compacts expand and degrade over time. Sandwiching metal hydride particles between metal plates to keep the particles in place and improve heat transfer has also been tried. Yet the plates further increase the overall weight of the storage device. They also restrict the volume expansion of the particles. Operational problems eventually develop as the swelling and contraction of the particles begins to affect hydrogen fluid flow and heat transfer through the plates. Performance ultimately degrades.